Various materials have been disclosed for use in optical information storage devices. Optical memories store information by changing some optical characteristic of the storage medium such as light absorbancy or reflectivity usually in response to a stimulus. Typically, the stimulus is a temperature change produced locally in the optical information storage material by illumination with a laser, electron, x-ray or other beam of energy. Because a recording and reading energy beam can be focussed into a very small spot, optical memories permit high density storage of information. The information placed in the optical memory material by a "writing" beam can be retrieved by a "reading" laser or other light source and an appropriate detector. For an example, see U.S. Pat. No. 3,505,658 to Fan et al.
Optical memories formed of chalcogenides, i.e. glassy materials composed of mixtures of germanium, arsenic, tellurium and silicon, vanadium pentoxide, etc., were disclosed in U.S. Pat. No. 3,271,591 to Ovshinsky et al. These optical memory materials are characterized by an increase in optical absorbancy, i.e. opacity, as they are heated resulting from a change from an amorphous state to a crystalline state.
The chalcogenide materials may be used as the basis of various memory devices. One example using germanium-selenium compounds was disclosed in U.S. Pat. No. 4,183,094 to Keezer et al. Germanium-selenium compounds undergo subtle microstructure changes in the thermally induced transition of optical properties. These changes occur at relatively low temperatures (e.g. 50 to 100.degree. C.) meaning that they can be difficult to control. The thermally induced shift in optical properties is relatively small, the band gap energy typically increasing only a few tenths of an electron-volt.
Amorphous silicon, germanium and silicon carbide were disclosed as optical memory materials in U.S. Pat. No. 3,716,844 to Brodsky. These materials also show a temperature dependent amorphous-to-crystalline transition, but their opacity decreases during that transition, in contrast to the chalcogenides. The thermally induced change in opacity typically represents change in band gap energy of less than one electron-volt.
When a material makes a transition from an amorphous to a crystalline state its density changes. That transition has a profound effect on the mechanical stability of the material and may even be considered a failure of the material. It is desirable to avoid such a transition in recording information in an optical memory in order to increase reliability.
It is an object of this invention to employ a material for optical information storage that has a thermally induced optical transition occurring at a sufficiently high temperature to ensure good thermal stability and in which a relatively large change in band gap energy is produced to ensure accurate readability of the stored information. This object is achieved by the use of multi-component films exhibiting transitions that do not involve the crystallization of the recording material.